November/December 2009

Weather Queries

by Tom Schlatter

What is a heating degree day and a cooling degree day?

Chris Keller Locke, New York

The concept of degree days arose when heating, ventilation, and air conditioning (HVAC) engineers wanted an index related to the energy required to heat and cool buildings based upon readily available temperature data. A degree day is simply the difference between the average daily temperature (usually taken to be the average of the maximum and minimum) and a base or reference temperature. For heating degree days, the base temperature is 65°F. The supposition is that people will want to heat their homes if the average daily temperature falls below 65°F. Thus, if the temperature today ranges from 40° to 60°, the average is 50°, and the climate record would show 15 heating degree days. If the average daily temperature is 65°F or higher, no heating degree days are recorded.

For cooling degree days, I was surprised to find disagreement about the base temperature. NOAA climate data publications have used a base of 65°F for years. Most sites on the Web are consistent with this. Yet, no less an authority than the Glossary of Meteorology defines cooling degree days with a base temperature of 75°F. Thus, if you are using cooling degree day information as a way to compare energy bills from different summer months, be sure you know which base temperature applies. If the average daily temperature is at or below the base temperature, no cooling degree days are recorded.

Like kilowatt hours on your electricity bill, heating and cooling degree days accumulate throughout the year. You should notice a good correlation between the accumulated heating degree days for winter months and your winter energy bills, and between the accumulated cooling degree days for summer months and your summer energy bills. The correlation won't be perfect for several reasons: 1) your total energy bill covers more than heating and air conditioning; 2) more solar radiation passes through your windows and is absorbed by the walls and roof of your home on sunny than on cloudy days, even if the average outside temperature is the same; and 3) the amount of insulation and weather stripping in your home determines how much energy passes between it and the outdoors when inside and outside temperatures differ, especially when the wind blows.

On the afternoon of November 28, 2008, a dark, elongated, featureless cloud passed overhead. Ice somewhat larger than pea-size fell with a temperature of 40°F. Some snow crystals seemed to adhere to the ice. Several National Weather Service spotters reported hail. Was this really hail? I didn't hear thunder. Could it have been sleet?

Tom Adams Marblehead, Massachusetts

The precipitation was probably neither hail nor sleet but rather graupel, also called snow pellets. Your question provides a good opportunity to review the conditions under which mixed precipitation (solid and liquid) forms. I—ll discuss stratiform precipitation first, then convective precipitation.

Stratiform precipitation is steady, not showery. It forms in clouds when the updrafts are gentle and uniform over a large area. The figure illustrates three scenarios of wintertime precipitation. It gives a vertical cross section of the atmosphere through a warm front, oriented so that south is on the left and north on the right. The dividing line between the light blue shading on the bottom and light red shading on the top marks the boundary between colder air near the surface and warmer air just above the frontal boundary. An inversion (increase of temperature with altitude) lies at the top of the cold air. The vertical axis is altitude from sea level to 3.0 km (about 9,800 ft). The labels (a), (b), and (c) are for 3 temperature profiles plotted in red, each centered on the freezing point, 0°C. The horizontal axis gives the temperature scale from −5 to +5°C. Note the inversions on each profile at the top of the cold air.

When freezing rain occurs at the ground, as in (a), the precipitation high in the clouds starts as snow. As it falls into a layer of air above freezing, it melts, becoming all liquid. Then it falls into a shallow layer of sub-freezing air near the ground. As soon as the drops strike any solid object they freeze, forming glaze ice. Aircraft flying in the shallow cold air will experience icing.

When sleet (ice pellets) occurs at the ground, as in (b), the situation is similar to that for freezing rain. The only difference is that the depth of the low-level cold air is greater. The temperature might also be lower. In either case, raindrops entering this layer freeze before reaching the ground. Sleet is easy to identify. It falls with a delicate tinkling sound. It consists of clear ice, sometimes with a tiny spicule squeezed out on its surface. The droplet freezes from the outside in. Ice occupies more volume than liquid water, so as the freezing approaches the center of the droplet, the pressure buildup cracks the outer surface, and a tiny squirt of water might freeze on the outer surface of the sleet particle. Your description of the ice particle rules out sleet.

When the atmospheric temperature profile lies entirely below freezing, as in (c), snow that forms in the clouds can reach the ground without melting. You saw little balls of ice, not recognizable snow crystals, and so we can rule out snow—which brings us to convective precipitation.

Conditions ripe for mixed stratiform precipitation in winter.

Convective precipitation is showery. It forms in clouds where the updrafts are fairly strong (measured in meters per second rather than centimeters per second) and localized. In all thunderstorms and in most convective clouds, ice crystals and supercooled cloud droplets (droplets at temperatures below freezing) coexist. The crystals, being more massive than the droplets, fall faster than the droplets. Collisions between droplets and a crystal result in the droplets freezing to the crystal, a process called riming. The crystal may collect so many droplets as to become unrecognizable, frequently assuming the shape of a rounded, sometimes conical white pellet. The white color is caused by air trapped in the little ball of low-density ice, called graupel or snow pellets. The most common size range is from ¼ to ⅜ inch. Your pea-size ice balls were almost surely graupel. Graupel has a substantial fall speed (2–3 meters per second), and so it must have survived the journey from cloud base to ground, where the temperature was 40°F, before melting. Evaporation from the surface of the graupel below cloud base causes cooling and forestalls melting.

If the ice balls are larger, denser, and especially if they contain any layers of clear ice, it's hail. Hail starts growing as graupel. If the updrafts are strong enough to suspend the particle for many minutes, and if the supply of supercooled cloud droplets is plentiful, then the ice ball can sometimes grow to dangerous proportions. Layers of clear ice form when the accretion rate of droplets is so large that water coats the entire surface of the growing hailstone before freezing. Hail rarely falls at surface temperatures near or below freezing, but it can happen when “elevated” thunderstorms form—that is, when a relatively warm, humid, and unstable air mass rides over the top of a shallow layer of cold air near the ground.

Weatherwise Contributing Editor THOMAS SCHLATTER is a retired meteorologist and volunteer at NOAA's Earth System Research Laboratory in Boulder, Colorado. Submit queries to the author attn: Weatherwise; Taylor & Francis LLC; 325 Chestnut Street, Suite 800; Philadelphia, Pennsylvania, 19106; or by email to ww@taylorandfrancis.com. Tom Schlatter is soliciting information on occurrences of deep hail (5 inches deep or more) in preparation for a feature article for the May-June 2010 issue of Weatherwise. If you have good quality photos of deep hail and can specify when and where the hail fell, how deep it was, and any other details, please send them to Margaret Benner at margaret.benner@taylorandfrancis.com.